This invention relates to an X-Ray tube which can produce a high brightness with a small focal spot and can be used over a wide operating range.
When using an X-ray tube for taking an X-ray photograph, it is necessary to minimize the size of the focal spot (electron beam spot) of the X-ray tube and to increase the tube current coming into the focal spot, that is, to increase the brightness. Thus, it has long been sought in the art of X-ray tubes to provide a small focal spot, of less than 0.1 mm and at the same time a tube current having a current density of more than twice that of the prior art.
FIG. 1 shows schematically cathode and anode (or target) electrodes of a prior art X-ray tube. As shown, heating current is passed through a helically wound filament coil 2 of a cathode electrode 1 to emit electrons and the electron beam standing for the X-ray tube current is focused by a focusing electrode 3 disposed about the filament to form a focal spot of a predetermined dimension on the surface of an anode or target electrode 4 opposing the cathode electrode 1. With such a prior art construction, so-called main focal spot having a diameter of A and an auxiliary focal spot having a diameter of B are formed on the surface of the target electrode 4 so that it has been extremely difficult to concentrate all electrons emitted from the filament on a small area having a diameter of less than 0.1 mm. The main focal spot is due to a group of electrons emitted from the front surface of the filament confronting the target electrode 4 whereas the auxiliary focal spot is due to a group of electrons emitted from the side surface of the filament 2. The main focal spot and the auxiliary focal spot have opposite behaviors with respect to the parameters that determine the foci. As a consequence, the current density distribution in the foci localizes at opposite ends of the main and auxiliary foci, thus producing four peaks or two peaks (the latter being formed when the main and auxiliary foci coincide with each other). Although it is possible to form a focal spot of less than 0.1 mm by concentrating either one of the main and auxiliary foci to one spot, in such a case the diameter of the other focal spot is broadened, resulting in a three peak distribution.
It will be understood from the foregoing that with the prior art cathode and anode arrangement it is difficult to obtain an extremely small focal spot. As a measure for eliminating the auxiliary focal spot, a cathode electrode structure has been proposed, as disclosed in Japanese Patent Application Laid Open No. 30292/'78, wherein a cathode electrode having one end divided into a main portion and side portions on both sides of the main portion is disposed in a step shaped groove formed in a focusing electrode and a filament is provided beneath the main portion to heat the same. According to this construction, it is possible to eliminate the auxiliary focal spot and to adjust the degree of electron focusing by a variable voltage source connected between the focusing electrode and the main portion. However, as far as prior art focusing electrodes as shown in FIGS. 3 and 4 of the aforementioned laid open patent specification and like the prior art of FIG. 1 of the instant application are concerned, since the image of an electron emitting region is focused on the surface of the anode electrode as will be discussed later in connection with FIG. 5 of the instant application, it is necessary to make extremely small the electron emitting region of the cathode electrode in order to make the diameter of the focal spot be less than 0.1 mm. However, such a construction decreases the magnitude of the X-ray tube current, thus limiting the field of application of the X-ray tube.
Another method of decreasing the effect of the auxiliary focal spot is disclosed on page 518 of the Journal of the Japanese Radiation Technical Society, 1977 in which the lens action of the focusing electrode is strengthened to reduce the diameter of the main focal spot below 0.1 mm, as in the well known zero bias type cathode electrode structure and the electron emission from the side surface of the filament which forms the auxiliary focal spot is restricted by a space charge effect so as to sufficiently increase only the main focus current density. This method, however, relies upon the space charge effect so that a desired small focal spot can be obtained only in a range of specific operating conditions. Thus, for example, when the operating voltage or tube current is varied, it is often impossible to obtain a desired small focal spot.
In the prior art fine focus X-ray tubes, since the lens action of the focusing electrode is generally strengthened, the electrons are liable to be influenced by the space charge effect so that the electrons emitted from the filament are difficult to flow towards the anode electrode with the result that the tube current supplying capability of the cathode electrode is limited to a relatively small value relative to the current receiving capability of the anode electrode, thereby degrading the quality of the X-ray tube and limiting the field of application thereof.